Outflow boundry tornados (arrrgg!)

[Kem Poyner]

First I must admit I suck at forecasting. I rely heavily on SPC and Stormtrack to pick my chase targets. On Saturday the 24th, I and hundreds of other chasers were on the Kansas/ Nebraska border watching towers build and croak while a tornadic beast was down in North Central Oklahoma. I think that storm was from an outflow boundry from Fridays storms? Reminded me of a fews years back when I got burned with the Jerral tornado event.
I noticed the chasers who did catch Saturdays Oklahoma storm was either not planning on chasing that day, or lived nearby. Even the SPC threw up a watchbox and hatched area only after the storm started.
My question is, is it possible to forecast well enough to catch an event like this? Do you ever toss aside usual dynamics and head South a couple hundred miles for this senerio?

 

[Chris C Sanner]

I can honestly say I woke up Saturday morning planning on going north into Kansas but after seeing the outflow boundary draped across NC Oklahoma it was an easy choice at that point. I'm a huge fan of outflow boundaries when there is adequate instability. What you may lack in shear, the boundary will make up for it a lot of times. The play, at least to me, was really obvious on Saturday. I targeted the Outflow Boundary/Front intersection and planned for Tornadoes. I'm not the best person to speak in meteorological terms but what I look for is high to extreme instability, at least adequate shear and a place where forcing is strongest to get a storm going.

Maybe someone else can explain it better, as I know there were several of us that targeted the Enid area out of preference.

 

[Patrick Marsh]

I noticed the chasers who did catch Saturdays Oklahoma storm was either not planning on chasing that day, or lived nearby. Even the SPC threw up a watchbox and hatched area only after the storm started.

I'm not so sure that it wasn't forecastable. I know of several chasers who were up in Kansas and instead of heading north decided to play Oklahoma.

My question is, is it possible to forecast well enough to catch an event like this? Do you ever toss aside usual dynamics and head South a couple hundred miles for this senerio?

The dynamics were in place for supercell thunderstorms so I'm not sure that you can say that people who were south threw dynamics aside and were only playing an outflow boundary. The southern play had backed surface winds, greater than or equal to 35 kts of deep layer shear, and the presence of an outflow boundary.

A real important thing that people seem to forget is that in addition to "dynamics" you also need thermodynamics. You can have the best shear, but if you don't get a storm then so what? The southern play had rich moisture in place (low 70s dewpoints south of the outflow boundary). Additionally, the northern play was going to be moisture starved because the previous night's MCS wiped out the rich moisture - and this hampered convection in Kansas (and northward).

The big issue in Oklahoma was going to be initiation - which occurred at the intersection of the dryline and outflow boundary. Once the storm developed it grew into a supercell and then was fortunate enough to stay in the high helecity zone on the north side of the boundary.

 

[Kem Poyner]

outflow boundry

Do you actually see the outflow boundry line on satellite image? I do not think my WxWorx gives that small of detail.
It makes me feel better to know that there where those who chose Oklahoma over points north. I want to learn that skill!

 

[Kenny Drake]

If you want to be able to forecast an OFB then you need to forecast convection. The 4km WRF is trying to do that (not too successfully.) I think in the next 10 or so years we will see operational models at high enough resolutions to pinpoint to notable accuracy convection (and hence OFB's). Just for reference the supercell that moved east across northern parts of Twin Cities, MN was riding an OFB. Winds at my house (in Hugo) were out of the E and veered to the SE about an hour before the tornado hit. Winds in St. Paul (to the south about 20 miles) were out of the south almost the entire day. If the convection that occurred during the early afternoon that moved NE'ward about 50-100 miles north of the Twin Cities didn't occur then more than likely neither would the tornado. The key to OFB's is their orientation. The OFB must be oriented such that a supercell will move parallel to it (but not into it.) If anyone remembers the El Reno, OK day (end of April... 2007?) everyone was in northern Oklahoma playing an OFB and so was I. The problem was the cells would go up and move into the cold pool and gust out. Sometimes OFB's are easy to pick out (like the Hugo, MN) case and sometimes they are not. Looks for the cold pool via ASOS data and it usually will show up. When you are in the field near an OFB you just need to keep an eye on the wind direction, T and DWPT. In general OFB's, if oriented correctly, are great for 2 reasons: (1) Focal point for convective initiation, and (2) Enhanced 0-1km SRH.

This is why skilled chasers won't pick a target until the morning of the case day if they know crapvection will be around. OFBs can be your best friend or your worst enemy. More often than not OFBs will be your worst enemy, but in some cases they are a blessing. If you are hoping for a great storm in a low shear environment than an OFB can bump up the 0-1km SRH a LOT. The Hugo, MN tornado on 5-25 was a perfect example of this.

http://www.rap.ucar.edu/weather/surface/2008052520_metars_dlh.gif

Note the winds near and just north of MSP (E. MN.)

Here is the pre-storm convection that caused it:
http://www.rap.ucar.edu/weather/rad...=black&endDate=20080525&endTime=16&duration=0

Tornadic supercell riding the boundary, enjoying enhanced SRH (SPC Mesoanalysis had 0-1km SRH ~200-300 along the boundary.)
http://www.rap.ucar.edu/weather/rad...=black&endDate=20080525&endTime=22&duration=0

Vis. Sat showing the cold pool:
http://www.rap.ucar.edu/weather/sat...=large&endDate=20080525&endTime=20&duration=0
Note the cloud-deck highlighting where the cold pool is. Note the clearing and Cu field build up just to the south of it. You can kind of make out the west-east'ish line of clouds marking the southern part of the OFB. The beginnings of the supercell can be seen to the WNW of the Twin Cities (KMSP). It initiated along the cold front-OFB intersection and "rode" it eastward.

 

[Chris Gullikson]

I agree this was a forecast-able event by mid-morning, but given the distance to the more obvious target, this event was unreachable by those already in Nebraska from the day before. I doubt anybody saw this coming the night before the event looking at model and observational data. I was just hoping to catch some tail end stuff in northern KS, I will gladly admit luck was on my side for this event.

 

[Greg McLaughlin]

Outflow boundaries often enhance horizontal vorticity that a storm can ingest into its updraft subsequently enhancing tornado potential. This is dependant on whether or not the storm can root along the boundary. Justin Teague and I had spent the night in Cherokee, OK and were planning to return to Tulsa the afternoon of May 24th. We got into Enid, OK around noon to eat and decided to look over the forecast. Noticing the outflow boundary draped roughly along the 412 corridor and 3000+SBCAPE already in place across central and western OK we decided to chase. To make a long story short we had a field day tracking the quickest cycling supercell we have ever witnessed.

We also chased the storms in southcentral KS on Memorial Day as they tracked along the outflow boundary. The early storms developed south of the boundary and moved north-northeast over the boundary becoming undercut by it. The storm near Pratt was the only cell to briefly root along the boundary in KS and therefore produced a tornado.

When forecasting storms along an outflow boundary it is important to find an area where the boundary is oriented in such a way that storms that develop along or near it will have a chance to root along the boundary. Often times you can have several storms develop and move over the boundary without producing tornadoes. When you get a storm that deviates from the mean flow as it interacts with the boundary the storm often becomes tornadic. The supercell on May 24th was such a storm. Justin and I chased the first storm that never rooted along the boundary, and once the storm to the south deviated we knew we had our storm. Anyway, I rambled longer than I intended, but hopefully for good cause.

The short of it is this.....if you have an outflow boundary that has stalled out or is SLOWLY drifting and is oriented in such a way to allow storms to root along it, you often end up with a tornado racetrack so to speak. Often times the only reason you get tornadoes on days like May 24, 2008 June 13, 2007 May 27, 1997 May 1, 2003 is because of outflow boundaries providing enhanced vorticity that would otherwise be abscent. Then there are days like April 26, 1991 where an outflow boundary ends up enhancing an already volitile situation and you get a monster outbreak of long-track, violent tornadoes.

 

[Shane Adams]

First I must admit I suck at forecasting. I rely heavily on SPC and Stormtrack to pick my chase targets. On Saturday the 24th, I and hundreds of other chasers were on the Kansas/ Nebraska border watching towers build and croak while a tornadic beast was down in North Central Oklahoma. I think that storm was from an outflow boundry from Fridays storms? Reminded me of a fews years back when I got burned with the Jerral tornado event.
I noticed the chasers who did catch Saturdays Oklahoma storm was either not planning on chasing that day, or lived nearby. Even the SPC threw up a watchbox and hatched area only after the storm started.
My question is, is it possible to forecast well enough to catch an event like this? Do you ever toss aside usual dynamics and head South a couple hundred miles for this senerio?

I'm sure some of you will scoff, but I was expecting a tornado day Sat in OK....just not to the magnitude we had. Also, I wasn't prepared for it to be as early as it was, so I was caught off guard when they went up as early as they did. I had everything together by 11am to leave (that day just a vidcam and a paper map), and made trips to O'Reilly's & WAL-MART to pick up a new fuse for my cig lighter adapter and a new power inverter, so I could run my scanner. Well, the new fuse did nothing for my power loss, so as I was slamming sh*t around in my car and swearing every word in the book, I happened to hear, on FM radio, we were under a tornado watch. I had been watching the OK mesonet and SPC's mesoscale discussion regarding the OK event, and knew cells had fired early, but this took me by surprise as I figured the storms would need a few more hours' heating to get the instability they needed to be tornadic.

So we blasted back home, I glanced at the radar to get a fix on where the storms were, grabbed my vidcam, and we took off. I was debating on the way to OKC whether to take I-35 north and get north more quickly, or to take the 240 swing and head up US81 to be further west....I figured with the slow storm speeds we needed to get to the storms as fast as possible, so we went with option #2. Once we had our route, I just watched the anvil through the low clouds and the closer we got, the clearer it became. It was classic old school style chasing. I live in central OK for a reason, and though it's been quiet around these parts for some years now, 2008 has shown that OK isn't the harlot many new chasers in the early 2000s have made it out to be.

I wasn't expecting to have the best chase of my career that day, but I was definitely planning to be chasing that area. Set-ups like May 24 are what I live for; insane CAPE + marginal shear + boundaries = slow storm speeds = dreams come true.

But if you asked me did I expect what happened to happen, the answer would be "no". I expected a tornado or maybe two.

 

[Greg McLaughlin]

Like you Shane I wasn't expecting convection to fire so early. After coming out of Wal-Mart in Enid around 1:30 and seeing that storm to the west I remember thinking, "this storm is going up way too early, but its on the boundary...it is a definate player". Justin and I decided to check it out. After that storm never produced we dropped to the southern storm once it deivated. We punched south through the core (yes we core-punched....and no I don't care what all those other people think about chasers who punch cores), and off to our southwest was a highly visible vaulted meso with a tornado underneath. We got down to just north of Lacy, OK on HWY 132 just as the first tornado lifted and the meso reorganized.

I am quite intrigued about how the meso cycled. The storm showed cyclonic and anticyclonic rotation side by side under the updraft. I believe we witnessed the updraft tilting horizontal vorticity (ala the outflow boundary) vertically and stretching it. Because the storm had rooted along the boundary and was ingesting surface parcels it had no problem cycling to tornadogenesis a second time as the cyclonic vorticicty became the dominant meso. The second tornado dropped, and we got right in behind it and with a Vx2100 and an HDdashcam, the rest is history.